1. Field of the Invention
The present invention relates to a polymeric dispersant having an affinity with a supercritical fluid, a dispersion composition comprising the polymeric dispersant, and a method of dispersing a dispersoid using the dispersion composition. More particularly, the present invention relates to a polymeric dispersant that can disperse a dispersoid in the scale of nanoparticles and maintain the dispersoid in a dispersed stable state by incorporating a compound having an affinity with a supercritical fluid into a polymeric dispersant such that the supercritical fluid more easily penetrates the openings in the dispersoid. The invention is also directed to a dispersion composition comprising the polymeric dispersant and to a method of dispersing a dispersoid by using the dispersion composition.
2. Description of the Related Art
The importance of nano-scale materials is increasing in various arts since the materials show very different properties compared to the same material having a large particle size.
An organic pigment among nano-scale particles is used for various purposes, such as ink, coating, toner, plastic, rubber, film and displays since they have excellent stability against heat and chemicals.
The chemical or physical properties of the organic pigment's nano particles depend on the shape of particles or crystals, the molecular orientation, the sizes and distribution, and the surface properties of the particles. The organic pigment's nano particles usually have diameters in the range of 10 to 50 nm, and can be easily aggregated into large particles due to their high surface energy.
Meanwhile, although a roll mill or a medium disperser, etc. has been previously used in order to disperse a solid-based dispersoid used as raw material for paint, ink, ceramic, cosmetics and food, etc., such apparatuses have long working times, and washing the apparatuses after working is cumbersome. The particles obtained by milling have a wide or broad particle distribution since the apparatuses generally impart mechanical shear force to the dispersoid particles to make the particulates.
Furthermore, dispersions employing fluid energy require high pressure compressed air and is effective for hard and fragile material. However, this process is less effective for soft particles.
To solve the problems described above, U.S. Pat. No. 5,921,478 suggests a method of minutely dispersing a dispersoid by mixing a solvent and a dispersoid, mixing a supercritical solvent therewith, allowing the supercritical solvent to be in a supercritical state through heating and compressing, and then reducing the pressure to atmospheric pressure.
The method disperses solid particles efficiently using the properties of a supercritical fluid that can change density quickly from that of gaseous state to that of liquid state by changing pressure and temperature.
Pure material is present in the state of gas, liquid or solid depending on pressure and temperature. The phase-change line between liquid-gas has a critical value of maximum temperature and pressure that the state of the liquid and the gas coexist at equilibrium condition. The gas and liquid have the same density above the critical value, and show single phase. Thus, a fluid, where gas phase and liquid phase coexist, refers to supercritical fluid where the temperature and pressure of the fluid are at above critical value.
The supercritical fluid has the same density and solubility as in a liquid state, and the same properties in diffusion or viscosity as in a gaseous state. Further, the supercritical fluid has low surface tension, more quickly penetrates than a liquid, and has solubility that is not present in a gaseous state.
A method of dispersing using the supercritical fluid is explained in more detail as follows. Firstly, a mixture of a dispersoid and a solvent is provided to a supercritical vessel, and a supercritical solvent is provided thereto. The supercritical solvent is converted from gaseous state to a supercritical fluid by heating and compressing, and the mixture and the supercritical fluid are mixed in the supercritical vessel. The supercritical fluid infiltrates into minute gaps of particles where the dispersoid are porous particles, and into the gap between the dispersoid particles. When high pressure of the vessel containing the supercritical fluid is drastically decreased, large pressure difference between the inside of the particles and inside the vessel occurs. The supercritical fluid that had been absorbed within particles induce high inner pressure within particles, and thus when outer pressure is drastically reduced, the supercritical fluid expands quickly. Finally, the dispersoid is ruptured to disperse in the scale of one micron or less.
The supercritical fluid is prone to getting wet in particulates and is quickly diffused into agglomerates of particulates since the fluid has a high diffusion coefficient over such a liquid solvent as water or alcohol, and has a low surface tension. Furthermore, the agglomerates of particulates are separated into individual particles to make primary particles. The dispersion of particulates is promoted since the interaction between the particulate and the supercritical fluid is greater than that between the particulates.
The supercritical fluid described above is a gas vaporized at room temperature, and includes carbon dioxide, nitrous oxide, ammonia, xenon, ethane, ethylene, propane, propylene, butane, isobutane, chlorotrifluoromethane, monofluoromethane, sulfur hexafluoride and mixtures thereof. Of these, carbon dioxide may be particularly preferred since it is less reactive, nontoxic and liquefiable at proper temperature.
One of the limitations in dispersions employing the supercritical fluid is the solubility of the supercritical fluid. That is, a nonpolar fluid such as carbon dioxide has low solubility over polar material. Accordingly, if the dispersoid is polar, the dispersion employing the supercritical fluid is not sufficiently produced. In other words, since the surface of the dispersoid particles is soaked or wetted with a solvent, the supercritical fluid cannot infiltrate easily into minute gaps of the particles. Thus, the impregnation of the supercritical fluid is largely performed by only molecular diffusion in a solvent, and even though a supercritical state is reached, the supercritical fluid cannot easily influence the gaps in the agglomerates of the dispersoid particles.
Further, the dispersoid particulates dispersed by employing the supercritical fluid may be reaggregated thereby deteriorating the dispersed state.
To solve the problems described above, a dispersant is further added to a system comprising a dispersoid, a solvent and a supercritical fluid. However, a general polymeric dispersant does not play a sufficient role as a dispersant since the dispersant has low affinity with a supercritical fluid.